Method and apparatus for measuring acoustic wave velocity using impulse response

ABSTRACT

The transit time of acoustic waves between a generator and a receiver positioned across a fluid chamber is determined by generating acoustic waves using a self-purging pneumatic sound generator, a transducer adjacent the outlet of the sound generator, and a receiving transducer positioned away from the sound generator outlet so that the acoustic waves received by the receiving transducer pass through a portion of the fluid. The electrical signals generated by the transmitting transducer and the receiving transducer are processed to obtain the impulse response of these electrical signals, and the point of maximum value is determined. This point of maximum value corresponds to the arrival time of the acoustic waves at the receiving location. The transit time determination may be used to calculate the fluid temperature or other parameters. The pneumatic sound generator is driven by a compressed air source so that the generator is automatically purged of any contaminants in the process of generating the random acoustic noise.

BACKGROUND OF THE INVENTION

This invention relates to the measurement of acoustic wave travel timein a fluid medium, with particular application to acoustic pyrometry.

Techniques are known for measuring the transit time of acoustic wavesfrom a transmitting location to a receiving location through a fluidmedium. Systems using both pulsed waves and continuous waves have beenproposed and used in the past for various purposes. In pulsed systems,the transit time is typically measured by noting the time differencebetween the generation of an acoustic pulse at the transmitting locationand the receipt of the same acoustic pulse at the receiving location. Incontinuous wave systems, the phase difference between the continuouswave at the transmitting location and at the receiving location providesan indirect measurement of the transit time. The transit time thusobtained is typically used to compute the velocity of the acoustic wavesin the medium. In acoustic pyrometry, the computed velocity is used tocompute the temperature of the fluid using a well-known relationshipbetween acoustic velocity and temperature. For a fuller discussion ofthe pulsed technique see M. W. Dadd, "Acoustic Thermometry In GasesUsing Pulse Techniques", High Temperature Technology, Vol. 1, No. 6,November, 1983. For a fuller discussion of the continuous wave techniquesee U.S. Pat. No. 4,215,582.

While both the pulsed and continuous wave techniques have been found tobe useful in many applications, each is demonstrably unsuitable inextremely noisy environments in which erroneous transit timedeterminations occur due to the masking presence of substantial noisesignals and multiple transmission paths for the acoustic wave. Oneexample of such a noisy environment is in the field of industrialboilers, such as modern utility boilers, chemical recovery boilers andrefuse boilers. Added to the noise problem is the compounding adverseeffect of attenuation of acoustic waves due to scattering of the wavesby temperature and velocity gradients (the latter in a moving fluid),and the masking effect of acoustic waves arriving at the receivinglocation via reflected boundary paths. While many efforts have been madeto improve the reliability of acoustic transit time measurement in noisyenvironments, such efforts have not met with success to date.

SUMMARY OF THE INVENTION

The invention comprises a method and system for measuring the transittime of acoustic waves between a transmitting location and a receivinglocation which is highly reliable in operation, even in the extremelynoisy and multi-path environments encountered in industrialapplications.

From a method standpoint, the invention comprises the steps oftransmitting acoustic waves through a fluid medium from a transmittinglocation to a receiving location, generating electrical counterpartsignals corresponding to the acoustic waves at the transmitting locationand at the receiving location, and determining the transit time of theacoustic waves between the transmitting location and the receivinglocation by obtaining the impulse response of the electrical signals anddetermining the point of maximum value corresponding to the arrival timeof the acoustic waves at the receiving location. The acoustic wavestransmitted through the fluid medium are random continuous or successivebursts each having a plurality of frequencies. For acoustic pyrometryapplications, the spectrum of interest is in the band of frequenciesbetween about 100 Hz and about 3,000 Hz.

The transit time value can be used to determine a number of parameters,such as the acoustic wave velocity in the fluid medium, the velocity ofthe medium itself (for a moving medium), and the temperature of thefluid medium.

From a system standpoint, the invention comprises a transmittertransducer for generating random acoustic waves for transmission throughthe fluid medium from a transmitting location to a receiving location,means for producing electrical signals corresponding to the acousticwaves generated by the transducer, a receiver transducer for sensing theacoustic waves arriving at the receiving location and for producingelectrical signals corresponding to the received acoustic waves, andcomputing means for receiving the electrical signals from the producingmeans and the receiver transducer and for determining the transit timeof the acoustic waves between the transmitting location and thereceiving location by obtaining the impulse response of the electricalsignals and determining the point of maximum value corresponding to thearrival time of the acoustic waves at the receiving location. Thetransmitting transducer is preferably a pneumatic generator powered by asuitable compressed air source and operated in a time sequential fashionin order to generate continuous or successive bursts of random acousticwaves.

The invention has been found to provide particularly improved results inextremely noisy environments, such as those found in industrial boilerapplications, while at the same time providing the known advantagesattendant with non-invasive acoustic pyrometric techniques. Also, thepneumatic embodiment of the transmitting transducer provides automaticpurging of contaminants in the sound generating path without adverselyaffecting the ability of the system to obtain the transmit time.

For a fuller understanding of the nature and advantages of theinvention, reference should be had to the ensuing detailed description,taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the preferred embodiment ofthe invention applied to acoustic pyrometry;

FIG. 2 is a top plan view of the sound transmitter unit;

FIG. 3 is a side elevational view of the sound transmitter unit;

FIG. 4 is a rear elevational view of the sound transmitter unit with thecover opened;

FIG. 5 is a wiring diagram of the sound transmitter unit; and

FIG. 6 is a plot of the impulse response versus time for the system ofFIG. 1 applied to a boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings, FIGS. 1-5 illustrate a preferred embodimentof a system incorporating the invention. As seen in FIG. 1, a pair ofboundary walls 11, 12 partially define a volume 14 in which a fluid (notillustrated) of interest is located. In the specific example describedbelow, the fluid is a gas in an industrial boiler, and the parameter ofinterest is temperature of this gas in the volume 14. In order todetermine this temperature, the transit time of acoustic waves betweenthe boundary walls 11, 12 must be measured according to the invention.

For this purpose, a pneumatic sound generator 21 is mounted externallyof wall 11 in any convenient fashion. The pneumatic sound generator 21is a unit having two valves schematically indicated by elements 22, 23and is designed to produce random noise in response to the applicationof compressed air to an inlet 24 from a suitable source shown). Thecompressed air is released via valve 23 which comprises an electricallyoperated solenoid valve (shown in FIG. 4) and which opens and closes inresponse to control signals supplied by a controller/processor 25.Pneumatic source 21 is coupled to the interior volume 14 via a pipewaveguide 27 having a flared end 28 coupled to a stand-off pipe coupler29 received in a suitable aperture in wall 11. Check valve 22 (FIG. 4)prevents high pressure in volume 14 from entering the compressed airline and contaminating the compressed air conduit (or otherwiseaffecting adversely the sound generator 21).

The entire system is designed to produce random noise in the frequencyband of interest for the acoustic pyrometry application for two majorreasons: firstly, the frequency spectrum of paramount interest toacoustic pyrometry is the band of frequencies between 100 Hz and 3,000Hz; and secondly, it is desired to have as many frequencies as possiblegenerated in the spectrum of interest. Thus, the generator 21 causes aspectrum to be generated with acoustic energy distributed throughout thefrequency band of interest.

Adjacent the flared end 28 of the pneumatic sound generator 21 is atransducer 30, which is preferably a model 941 piezoelectric transduceravailable from Scientific Engineering Instruments, Inc. of Sparks, Nev.and which generates electrical signals corresponding to the actualacoustic waves generated by pneumatic generator 21 and injected into thevolume 14 via waveguide 27 and coupler 29. These electrical signals arecoupled via a cable 31, a preamplifier 32 and cable 26 to thecontroller/processor 25 and represent a function x(t) required for thesignal processing described below. Preamplifier 32 is a dual gainamplifier having a low gain operation and a high gain operation. The lowgain operation is used for measuring the high intensity transmittingsignal and the high gain operation is used for measuring a receivedsignal.

Adjacent boundary wall 12 is a second transducer 34 which issubstantially identical to transducer 30 and which generates electricalsignals corresponding to the acoustic waves which travel across volume14 and reach the region of boundary wall 12 adjacent transducer 34. Theoutput of transducer 34 is coupled via a cable 35 and a secondpreamplifier 36 as a second function y(t) to controller/processor 25.Preamplifier 36 is essentially identical to preamplifier 32 inconstruction and function, and is used as a high gain preamplifier whenused as a receiver amplifier for detecting acoustic; waves generatedwithin the volume 14 by sound generator 21. Similarly, preamplifier 36is used as a low gain amplifier when sound generator 41 is used as theacoustic wave generator for transmitting waves in the opposite directiontowards boundary wall 11.

The system shown in FIG. 1 is designed to be symmetric about thevertical plane through the middle of volume 14. Consequently, a secondpneumatic source 41, check valve 42, electrically operated air valve 44,pipe waveguide 47 and flared end 48 are provided as shown. It should beunderstood that such symmetry is not required for all applications, butonly those applications in which it is desired to present the capabilityof generating acoustic waves alternately in opposite directions acrossvolume 14.

The acoustic waves generated by source 21 or generator 41 for the highnoise acoustic pyrometry application comprise a constant flow or aseries of successive bursts of random acoustic waves in the frequencyband of interest.

The electrical counterparts to the generated acoustic waves developed bytransducer 30 and the electrical counterparts to the received acousticwaves generated by transducer 34 are coupled as functions x(t) and y(t),respectively to controller/processor 25 for further processing. Thisprocessing proceeds as follows.

An impulse response, h (τ) calculation is performed on the signals x(t)and y(t). The impulse response for this system is given as the inverseFourier Transform of the frequency response function, H(f), that: is,

    h(τ)=F.sup.-1 [H(f)]=F.sup.-1 [.sup.Sxy /S.sub.xx ]    (1)

where

S_(xx) (f)=averaged autospectral density function or autospectrum ofx(t), and

S_(xy) (f)=averaged cross spectral density function or cross spectrumbetween x(t) and y(t)

It is well known that the cross-correlation function between x(t) andy(t) is given by

    R.sub.xy (τ)=F.sup.-1 [S.sub.xy ]                      (2)

As can be seen from a comparison of the two equations, the unit impulseresponse function resembles a cross-correlation function for an inputx(t) with a uniform spectral density of S_(xx) (f)=1. In effect, theunit impulse response provides the cross-correlation function for auniform input spectral density. Hence, the input data can becomputationally pre-whitened over its frequency range using a unitimpulse response calculation, which improves the definition ofindividual propagation paths. This is an extremely important feature inacoustic pyrometry where multi-path signals are generated in the cavityof the furnace. In particular, since the actual input spectrum of theacoustic wave source extends over the relatively wide range noted aboveand has concentrations of power, improved resolution of flight paths canbe achieved by using the unit impulse response computation. As aconsequence, it is important to provide as many frequencies as possiblethroughout the frequency band of interest for acoustic pyrometry sinceeach frequency generates a concentration of power for use by the unitimpulse response calculation.

FIG. 6 illustrates the result of the impulse response computation forvalues of x(t) and y(t) obtained in a coal fired 265 megawatt utilityboiler presenting an extremely noisy environment. This data was obtainedwith a pneumatic source 21 capable of generating acoustic waves inexcess of 130 dB re: 20μPa @1 m. FIG. 6 is a plot of the magnitude ofthe impulse response function along the ordinate versus time along theabscissa. The results show a prominent peak at a value of 17.5milliseconds. Efforts to obtain the same pronounced data using a pulsedchirp system have been found to fail due to the level of noise in thefurnace and the presence of multipaths. Other experimental results haveestablished the advantages of the invention in obtaining reliable datain particularly noisy environments.

As will now be apparent, the invention provides a method and system forenabling the accurate determination of the transit time between twoboundary points in a bounded volume of acoustic waves. From this transittime measurement, the velocity of acoustic waves in the fluid mediumbetween the two boundaries can be computed, and the temperature andvelocity of fluid (e.g., gas) can also be computed from the velocitycomputation using a well known relationship. Further, due to the use ofa pneumatic acoustic generator 21 (and alternate, symmetric generator41), energy levels beyond those available from electromechanicaltransducers can be achieved, with a corresponding increase in theability of a system employing acoustic pyrometry to obtain reliabletransit time basic information. In addition, by providing transducer 30adjacent the entrance point of the acoustic waves into the volume 14, areliable electrical signal replica of the acoustic waves actuallyinjected into the volume 14 can be obtained for subsequent signalprocessing purposes; and an accurate replica of the received acousticwaves at the receiving wall boundary is obtained by the use oftransducer 34. Consequently, intermediate effects produced by pipe 27and flared end 28 are substantially reduced or eliminated from theinformation signals x(t) and y(t), which eliminates the necessity ofproviding compensation factors found in prior art devices using storedwaveforms.

One important aspect of the invention lies in the use of the pneumaticsound generator 21 as both an acoustic wave generation device and also acontaminant purging device. In known systems, for example, usingnon-pneumatic generators (such as electro mechanical devices,piezoelectric transducers and the like), in particularly contaminatedenvironments, the wave guides extending between the wave generatingelement (e.g., a diaphragm) and the entrance to the volume 14 can becomecontaminated with particulate matter found in the interior of the volume14 (such as soot) in a coal-fired boiler system. The buildup of thecontaminating particles over time leads to a change in the acousticcharacteristics of the sound generating system (and the acousticreceiving system as well). Consequently, these units require cleaning atmaintenance intervals whose frequency depends on a number of factorsaffecting the buildup of contamination. With the pneumatic soundgenerator described above, purging of the acoustic paths leading fromthe sound source to the volume under investigation is automaticallyperformed along with the generation of the acoustic waves. Theimportance of this advantage is commensurate with the rate at whichcontamination accumulates in the system subject to the acoustic testing.For relatively clean environments, either the pneumatic generatordescribed above or conventional acoustic wave generating devices (suchas those discussed in the references cited above) may be employed.

An another significant advantage of the invention is that the effect ofincreasing levels of noise, which tend to mask the transit timeinformation, can be compensated for by either increasing the length oftime during which the acoustic waves are generated by the transmitterand detected by the receiver or by increasing the number of averages inthe frequency domain, especially by providing an increased number ofburst repetitions and corresponding impulse response computations whenusing the burst mode.

While the above provides a full and complete disclosure of the preferredembodiment of the invention, various modifications, alternateconstructions and equivalents will appear to those skilled in the art.For example, other specific frequencies may be employed in both acousticpyrometry applications and other applications. Also, other transducersthan those specifically identified with respect to elements 30, 34 maybe employed, as desired. Therefore, the above descriptions andillustrations should not be construed as limiting the invention which isdefined by the appended claims.

What is claimed is:
 1. A method of measuring a transit time of acousticwaves in a fluid in a noisy environment, said method comprising thesteps of:(a) transmitting randomly generated acoustic waves through thefluid from a transmitting location to a receiving location; (b) couplingelectrical signals corresponding to said acoustic waves transmitted fromsaid transmitting location to a computing device; (c) couplingelectrical signals corresponding to the acoustic waves arriving at thereceiving location to the computing device; and (d) determining thetransit time of the acoustic waves between the transmitting location andthe receiving location by(1) obtaining the impulse response from theelectrical signals corresponding to said transmitted waves and from theelectrical signals corresponding to the received waves, whereby saidimpulse response is defined by the relationship:

    h(τ)=F.sup.-1 [S.sub.xy /S.sub.xx ]

wherebyx(t)=the electrical signals corresponding to said transmittedwaves y(t)=the electrical signals corresponding to said received wavesX(f)ΔF[x(t)] Y(f)ΔF[y(t)] S_(xy) ΔX*Y S_(xx) ΔX*X and (2) determiningthe point of maximum value of the impulse response, the transit time ofthe acoustic waves being represented by the value τ at which the pointof maximum value of the impulse response is found.
 2. The method ofclaim 1 further including the step of computing the temperature of thefluid from the transit time determined in step (d).
 3. The method ofclaim 1 wherein said step (a) of transmitting includes transmittingcontinuous acoustic waves.
 4. The method of claim 1 wherein said step(a) of transmitting includes transmitting successive bursts of randomacoustic waves.
 5. The method of claim 1 wherein said step (a) oftransmitting includes transmitting random acoustic waves having aspectrum of interest and a plurality of frequencies.
 6. The method ofclaim 5 wherein the spectrum of interest is a band of frequenciesbetween about 100 Hz and 3,000 Hz.
 7. The method of claim 1 furtherincluding the step of computing the velocity of the acoustic waves inthe fluid from the transit time determined in step (d).
 8. A method ofdetermining the temperature of a fluid in a noisy environment, saidmethod comprising the steps of:(a) transmitting successive bursts ofrandom acoustic waves through the fluid from a transmitting location toa receiving location, said waves having a spectrum of interest and aplurality of frequencies; (b) coupling electrical signals correspondingto said acoustic waves transmitted from said transmitting location to acomputing device; (c) coupling electrical signals corresponding to theacoustic waves arriving at the receiving location to the computingdevice; and (d) determining the transmit time of the acoustic wavesbetween the transmitting location and the receiving location by(1)obtaining the impulse response from the electrical signals correspondingto said transmitted waves and from the electrical signals correspondingto the received waves, whereby said impulse response is defined by therelationship:

    h(τ)=F.sup.-1 [S.sub.xy /S.sub.xx ]

wherebyx(t)=the electrical signals corresponding to said transmittedwaves y(t)=the electrical signals corresponding to said received wavesX(f)ΔF[x(t)] Y(f)ΔF[y(t)] S_(xy) ΔX*Y S_(xx) ΔX*X and (2) determiningthe point of maximum value of the impulse response, to transit time ofthe acoustic waves being represented by the value τ at which the pointof maximum value of the impulse response is found and (e) computing thetemperature of the fluid from the transmit time determined in step (d).9. The method of claim 8 wherein the spectrum of interest is a band offrequencies between about 100 Hz and about 3,000 Hz.
 10. A system formeasuring a transit time of acoustic waves in a fluid in a noisyenvironment, said system comprising:transmitter transducer means forgenerating random acoustic waves for transmission through the fluid froma transmitting location to a receiving location; means for producingelectrical signals corresponding to the acoustic waves generated by saidtransmitter transducer means; receiver transducer means for sensing theacoustic waves arriving at the receiving location and for producingelectrical signals corresponding to the received acoustic waves; andcomputing means for receiving the electrical signals from said producingmeans and said receiver means and for determining the transit time ofthe acoustic waves between the transmitting location and the receivinglocation by(1) obtaining the impulse response of the electrical signalscorresponding to said transmitted waves and from the electrical signalscorresponding to the received waves, whereby said impulse response isdefined by the relationship:

    h(τ)=F.sup.-1 [S.sub.xy /S.sub.xx ]

wherebyx(t)=the electrical signals corresponding to said transmittedwaves y(t)=the electrical signals corresponding to said received wavesX(f)ΔF[x(t)] Y(f)ΔF[y(t)] S_(xy) ΔX*Y S_(xx) ΔX*X and (2) determiningthe point of maximum value of the impulse response, transit time of theacoustic waves being represented by the value τ at which the point ofmaximum value of the impulse response is found.
 11. The system of claim10 wherein said computing means further includes means for determiningthe temperature of the fluid medium from the transit time.
 12. Thesystem of claim 10 wherein said transmitter transducer means includesmeans for generating continuous random acoustic waves.
 13. The system ofclaim 10 wherein said transmitter transducer means includes means forgenerating successive bursts of random acoustic waves.
 14. The system ofclaim 10 wherein said transmitter transducer means includes means forgenerating acoustic waves having a spectrum of interest and a pluralityof frequencies.
 15. The system of claim 14 wherein the spectrum ofinterest is a band of frequencies between about 100 Hz and about 3,000Hz.
 16. The system of claim 10 wherein said computing means furtherincludes means for determining the velocity of the acoustic waves in thefluid medium from the transit time.
 17. A system for determining thetemperature of a fluid in a noisy environment, said systemcomprising:transmitter transducer means for generating successive burstsof random acoustic waves for transmission through the fluid from atransmitting location to a receiving location, said bursts having aspectrum of interest and plurality of frequencies; means for producingelectrical signals corresponding to said acoustic waves generated bysaid transmitter transducer means; receiver transducer means for sensingthe acoustic waves arriving at the receiving location and for producingelectrical signals corresponding to the received acoustic waves; andcomputing means for receiving the electrical signals from said producingmeans and said receiver means and for determining the transit time ofthe acoustic waves between the transmitting location and the receivinglocation by(1) obtaining the impulse response of the electrical signalscorresponding to said transmitted waves and of the electrical signalscorresponding to the received waves, whereby said impulse response isdefined by the relationship:

    h(τ)=F.sup.-1 [S.sub.xy /S.sub.xx ]

wherebyx(t)=the electrical signals corresponding to said transmittedwaves y(t)=the electrical signals corresponding to said received wavesX(f)ΔF[x(t)] Y(f)ΔF[y(t)] S_(xy) ΔX*Y S_(xx) ΔX*X and (2) determiningthe point of maximum value of the impulse response, the transit time ofthe acoustic waves being represented by the value τ at which the pointof maximum value of the impulse response is found; said computing meansfurther including means for determining the temperature of the fluidmedium form the transit time.
 18. The system of claim 17 wherein thespectrum of interest is in the band of frequencies between about 100 Hzand about 3,000 Hz.
 19. The system of 15 wherein said transmittertransducing means includes a pneumatic driver unit.